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United States Patent |
6,071,380
|
Hoffman
|
June 6, 2000
|
Method of papermaking having zero liquid discharge
Abstract
A method of papermaking having zero liquid discharge. A pulp slurry is
produced from recycled cellulosic material and the slurry contains
suspended cellulosic fiber material, solid contaminants and dissolved
water soluble solids. The pulp slurry is subjected to a cleaning operation
to produce a first flow stream containing the cellulosic fiber material
and dissolved solids and a second flow stream containing dissolved solids
along with solid contaminants. The first flow stream is washed with water
to remove a substantial portion of the dissolved solids and provide a
washed pulp which is used in a papermaking machine to produce a paper
sheet. The second flow stream from the cleaning operation is compacted to
produce a solid residue that can be landfilled and a liquid residue
containing dissolved solids that can be recycled to the pulp cleaning
operation. In a modified form of the invention, the liquid residue from
the compaction can be subjected to a process, such as evaporation, to
provide a solid residue and a purified water or condensate which can be
used in the papermaking machine.
Inventors:
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Hoffman; Roger P. (Green Bay, WI)
|
Assignee:
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Hoffman Environmental Systems, Inc. (Green Bay, WI)
|
Appl. No.:
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037250 |
Filed:
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March 9, 1998 |
Current U.S. Class: |
162/190; 159/47.3; 162/29; 162/44; 162/47; 162/55; 162/60; 210/650; 210/652 |
Intern'l Class: |
D21F 001/66; D21C 009/02; D21C 001/70 |
Field of Search: |
162/190,264,55,29,47,30.1,30.11,60,43,44,290,189
159/47.3
210/650,651,652
|
References Cited
U.S. Patent Documents
2771823 | Nov., 1956 | Lukemire.
| |
3467573 | Sep., 1969 | Vogel.
| |
3884755 | May., 1975 | Frost, III.
| |
3988200 | Oct., 1976 | Smith, Jr.
| |
3988201 | Oct., 1976 | Smith, Jr.
| |
4115188 | Sep., 1978 | O'Brien et al.
| |
4574032 | Mar., 1986 | Ringley.
| |
5302246 | Apr., 1994 | Nykanen et al.
| |
5380402 | Jan., 1995 | Ryham et al.
| |
5449437 | Sep., 1995 | Vikio | 162/190.
|
5470481 | Nov., 1995 | Modell et al.
| |
5567278 | Oct., 1996 | Meinander | 162/190.
|
Other References
Jonsson, B.M. "Advanced Water Recycling System Required for New South
African Mill", in Trends and Developments in Papermaking edited by John C.
W. Evans, Miller and Freeman Publications, Inc., Chapter 15, pp. 60-64.
Guss, Daniel B., "Closed Water Systems in Mills Using Secondary Fiber",
TAPPI, vol. 61, No. 6, pp. 19-21.
|
Primary Examiner: Fortuna; Jose
Attorney, Agent or Firm: Andrus, Sceales, Starke & Sawall
Parent Case Text
This application is a continuation-in-part of application Ser. No.
08/654,248, filed May 28, 1996 now U.S. Pat. No. 5,762,758, which in turn
is a continuation of application Ser. No. 08/298,748, filed Aug. 31, 1994,
now abandoned.
Claims
I claim:
1. A method of papermaking have zero liquid discharge, comprising the steps
of pulping recycled waste paper and recycled water from a papermaking
operation with pulping chemicals to form a pulp slurry, said recycled
water containing dissolved water soluble solids and said waste paper
containing cellulosic fiber material, solid contaminants, and water
soluble solids, subjecting the pulp slurry to a cleaning operation to
produce a first flow stream containing said cellulosic fiber material and
dissolved solids and being substantially free of said solid contaminants
and a second flow stream consisting primarily of suspended solid
contaminants and dissolved solids, washing said first flow stream with
water to remove residual dissolved solids from said first flow stream and
provide a washed pulp, utilizing the washed pulp in a papermaking machine
to provide a paper sheet, supplying water to the papermaking machine,
recovering water from the papermaking machine and utilizing the recovered
water in the pulp washing operation, recovering water containing dissolved
solids from the pulp washing operation and utilizing the recovered water
in the pulping operation, removing water from the second flow stream to
produce a solid residue and a liquid residue containing dissolved solids,
and utilizing the liquid residue in the pulp cleaning operation.
2. The method of claim 1, and including the step of utilizing water
containing dissolved solids recovered from the pulp washing operation in
the pulp cleaning operation.
3. The method of claim 1, and including the step of removing water from the
second flow stream by mechanical operation.
4. The method of claim 1, and including the step of discharging the solid
residue to a landfill operation.
5. The method of claim 1, and including the step of subjecting the liquid
residue to a purification treatment to produce a substantially pure water
phase and residual material, and incorporating the substantially pure
water phase in the papermaking operation.
6. The method of claim 5, and including the step of incorporating low
quality make-up water with said liquid residue in the purification
treatment to produce the substantially pure water phase.
7. The method of claim 5, wherein the purification treatment is
evaporation.
8. The method of claim 5, wherein the purification treatment is reverse
osmosis.
9. The method of claim 5, wherein the purification treatment is
ultrafiltration.
10. A method of papermaking having zero liquid discharge, comprising the
steps of pulping recycled cellulosic fiber material with water and pulping
chemicals to form a pulp slurry, said cellulosic fiber material containing
solid contaminants, subjecting the pulp slurry to a cleaning operation to
provide a first flow stream containing a cleaned pulp and dissolved solids
and being substantially free of said solid contaminants and a second flow
stream containing suspended solid contaminants and dissolved solids,
washing said first flow stream with water to remove residual dissolved
solids from said first flow stream and provide a washed pulp, utilizing
the washed pulp in a papermaking machine to provide a paper sheet,
supplying water to the papermaking machine, recovering water from the
papermaking machine and utilizing the recovered water in the pulping
operation, removing water from the second flow stream to produce a
compacted residue containing said solid contaminants and dissolved solids
and a liquid residue containing dissolved solids, and utilizing the liquid
residue in the pulp cleaning operation.
Description
BACKGROUND OF THE INVENTION
Most paper or paperboard operations discharge tremendous quantities of
liquid which contain suspended solids, such as wood fibers, clay, calcium
carbonate, titanium dioxide, sand, and wood bark, along with oxygen
demanding organic compounds, such as starches, lignins, and hemicellulose.
In addition, the liquid discharge can also contain trace chemicals, such
as chlorinated organic compounds, polychlorinated biphenols, metals such
as barium, aluminum, and other trace elements. The adverse environmental
impact of this liquid discharge from a papermaking operation is well
documented.
There are several reasons for the liquid discharge from a papermaking
operation. In some cases, the liquid discharge is due to careless,
excessive use of water and, if excess water is used, it must be discharged
from the operation. In other cases, the liquid discharge is the result of
poor water system design, which results in inadvertent use of fresh water.
In still other instances, excess water has been employed in an attempt to
keep the suspended and dissolved solid levels at a low enough level to
ensure optimum paper machine conditions. In this latter case, the added
water effects a reduction in suspended and dissolved solids, but produces
a corresponding increase in effluent flow.
Typically the most highly contaminated water discharged from a conventional
papermaking operation utilizing recycled waste paper, results from the
cleaning of the recycled pulp. The liquid residue from the cleaning
operation is normally subjected to floatation clarification, in which the
lighter weight solid material is recovered as a floating layer which can
be burned or landfilled. The liquid effluent from the clarification
process, which contains dissolved solids and oxygen demanding compounds,
then must be treated by either an aerobic or anaerobic process before it
can be discharged to the sewer system. Evaporation of the liquid effluent
has not been feasible due to the tremendous volume of effluent.
SUMMARY OF THE INVENTION
The invention is directed to a papermaking operation having zero liquid
discharge. In accordance with the invention, a recycled cellulosic pulp is
initially subjected to a cleaning operation. The cleaning operation in
itself is conventional, and normally includes a number of stages in which
various types of suspended solids or particulate material are removed from
the pulp. In a typical cleaning operation, debris and heavy material, such
as stones, metal, glass, and the like are initially removed from the pulp
by a liquid cyclone, and the pulp is then subjected to a coarse screening,
followed by a secondary finer screening to remove large and small size
contaminants. Following this, the pulp is subjected to a forward cleaning
operation utilizing a liquid cyclone to remove sand, small fiber bundles,
ink, and the like. Several stages of forward cleaning are normally
employed. The sand, dirt, and other solid material that is removed during
the forward cleaning is then subjected to compaction, and the liquid phase
from the compaction is discharged to a clarifier.
After the forward cleaning, the pulp is then passed through a slotted
screen cleaner, which removes smaller particles of plastics, hot melt,
adhesives, and the like. The solid residue from the slotted screen cleaner
is again compacted and liquid removed during the compaction is sent to the
clarifier.
Following the slotted screen cleaning the pulp is subjected to reverse
cleaning utilizing a liquid cyclone, in which lighter material, such as
wax, latex, hot melt, is separated from the heavy material, which is the
pulp. Again, the reverse cleaning is normally carried out in a number of
stages, and the separated light phase is sent to the clarifier.
In the clarifier, which is preferably a floatation clarifier, the solid
materials are separated as a floating upper layer or sludge which is
combined with the slotted screen and forward cleaner rejects and is
discharged to a mechanical solid waste dewatering unit, such as a sludge
press. A portion of the liquid effluent from the clarifier or from the
sludge press may be sent to an evaporator.
In the evaporator, the liquid effluent from the clarifier or sludge press
is heated by a low pressure steam discharged from the dryer section of the
papermaking machine. High pressure steam, at a pressure normally about 160
psig, from a steam generating plant is supplied to the dryers in the dryer
section of the papermaking machine and a mixture of low pressure steam at
a pressure of perhaps 50 psig, and steam condensate, are discharged from
the dryers to a condensate tank where the low pressure steam is separated
from the condensate. The low pressure steam is then delivered to the
evaporator and used as the best source to evaporate the liquid effluent
from the clarifier. The condensate from the dryer steam as well as the
condensate from the vaporized liquid effluent can be combined and
delivered to a water storage tank, or alternately the condensate from the
dryer steam, which is relatively pure, can be recycled to the steam
generating plant. The evaporated residue from the evaporator is
transferred to the solid waste dewatering unit where it is combined with
the sludge from the clarifier. The combined residues are dewatered in the
sludge press of the dewatering unit to produce a residue containing about
30 to 50% solids, and this material can then either be landfilled or
incinerated. The evaporator residue may be concentrated to the point that
it is crystallized or alternately the concentration of the dissolved
solids is elevated to such a level that the sludge press reaches an
equilibrium level where the pressed material carries with it the dissolved
solids at the rate at which they are entering the system.
The recycled pulp, after cleaning, is subjected to counter-current washing.
Typically, recycled pulp is not washed in a counter-current system. The
counter-current washer uses very little wash water and acts to remove the
majority of the dissolved solids in the pulp. A properly designed washer
with the appropriately segregated water system, will serve as a barrier
keeping high levels of ionic materials, such as alum; other soluble
materials, such as starches; and colloidal material, such as fiber debris,
calcium carbonate and clay, from getting into the paper machine water
system.
Following the washing, the pulp undergoes normal stock preparation and can
then be utilized in the papermaking process.
The condensed dryer steam and the condensate resulting from the evaporation
of the liquid effluent from the clarifier can be combined with fresh
makeup water in the water storage tank or chest, and this water is then
employed in the papermaking process to be used in portions of the process
that require relatively pure water with a minimum mineral content, such as
for example, the showers and pump sealing water. Water recovered from the
papermaking process is recycled back to the pulp washing operation and
used to wash the pulp, and similarly, water recovered from the washing
operation, can be recycled back and employed in pulp cleaning and pulping.
With the process of the invention, there is zero liquid discharge from the
papermaking operation, the only discharge being solid residue that can be
either landfilled or incinerated.
As a further advantage, the process of the invention produces a higher
fiber yield, as no small fibers or fines are lost in water discharge.
The process of the invention also requires less energy than conventional
papermaking processes. In a conventional process, water at an elevated
temperature of approximately of 120.degree. to 160.degree., is used as the
incoming water in the papermaking operation, and the water is normally
heated to this temperature either through direct heating or sparging with
steam. In the process of the invention, the only required heat source is
the heat needed for evaporation of the liquid effluent from the clarifier,
and the steam from that evaporation will meet the heat requirements for
the incoming water.
Further, the process of the invention saves the energy normally required in
the aerobic or anaerobic treatment of the liquid being discharged to the
sewer system.
As a further advantage, the process of the invention results in a
considerably lesser water consumption than normal papermaking operations.
This is an important factor when the papermaking mill is located in an
arid region of the country.
In a modified form of the invention, recycled cellulosic pulp slurry from
the pulping operation is subjected to a conventional cleaning operation
including the removal of heavy contaminants by centrifugal cleaners,
followed by coarse and fine screening, forward and reverse cleaning, and
the separation of lighter phases through floatation clarification. The
cleaning operation produces a first aqueous flow stream which contains
dissolved solids and the majority of the cellulosic fiber material and is
substantially free of solid contaminants, and a second reject flow stream
or sludge that consists of high levels of solid contaminants and dissolved
solids.
The cleaned cellulosic pulp in the first flow stream is then subjected to
countercurrent washing which serves to remove the majority of the
dissolved solids in the pulp slurry. Following the washing, the pulp
slurry undergoes normal stock preparation and then is utilized in a
papermaking machine to produce a paper sheet.
The second or reject flow stream from the cleaning operation and consisting
predominantly of the solid contaminants is subjected to a mechanical
dewatering operation to produce a solid cake which then can be discharged
to a disposal site, such as a landfill or incinerator. The water effluent
from the dewatering operation can be recycled and used in the pulp
cleaning operation.
As in the first embodiment, water recovered from the papermaking machine,
which can consist of water recovered from the showers and pulp seal water,
as well as water extracted from the wet paper web, is then recycled to the
pulp washer and used as wash water for washing the pulp. The water
recovered from the pulp washing operation, consisting of residual wash
water, as well as water recovered by virtue of the increase in consistency
of the pulp as it passes through the washing operation, is then recycled
to the pulping vessel for use in producing the pulp slurry. A portion of
the recirculated water from the washing operation can also be recirculated
to the pulp cleaner.
A primary concern in any attempt to develop a zero liquid discharge system,
is the progressive buildup of dissolved solids in the recirculated water.
However, with the method of the invention a substantial portion of the
dissolved solids are carried away in the solid sludges or residue which
can be landfilled or incinerated, thus preventing an undesirable buildup
of dissolved solids in the recirculated water.
The modified form of the invention does not require additional operations,
such as evaporation, reverse osmosis or ultrafiltration, to remove
dissolved solids from the water stream, but these additional operations
can enhance the overall system by providing more purified water for the
papermaking operation, thereby increasing the available water for washing.
Other objects and advantages will appear in the course of the following
description.
DESCRIPTION OF THE DRAWINGS
The drawing illustrates the best mode presently contemplated of carrying
out the invention.
In the drawings:
FIG. 1 is a flow diagram illustrating the process of the invention.
FIG. 2 is a flow diagram illustrating a modified form of the invention.
DESCRIPTION OF THE ILLUSTRATES EMBODIMENT
FIG. 1 is a flow diagram illustrating the papermaking process of the
invention having zero liquid discharge. A recycled pulp slurry is
initially produced in a pulping vessel 1 using conventional techniques.
The recycled material can include used corrugated containers, wastepaper,
white office waste, or the like, and can contain debris and solid
contaminates and also include a substantial quantity of fillers, such as
clay and calcium carbonate, as well as ink, which normally would not be
present in a virgin pulp.
In the pulping operation, a caustic material such as sodium hydroxide, is
added to the pulp, along with water, and conventional surfactants to wet
the fibers. Sodium silicate can be used as a deinking aid and hydrogen
peroxide as a brightener.
In the pulping vessel 1, the pulp is maintained at a temperature generally
in the range of 120.degree. F. to 160.degree. F., and is subjected to
agitation or a shearing action to break down the fiber bundles.
The pulping can either be a continuous operation, in which materials are
continuously fed and discharged from the pulper, or a batch pulping
operation. With a batch operation the pulp is maintained in the pulper at
the above-noted temperature and with agitation for a period generally in
the range of 15 to 35 minutes.
The consistency of the pulp in the pulping operation can vary depending
upon the type of paper being produced. When producing corrugated medium
for use in corrugated cartons, the pulp may be at a consistency in the
range of 3% to 5% by weight of solids. On the other hand, when producing
more sophisticated grades of paper, such white office paper, the pulp may
have a consistency of 10% to 15% by weight of solids.
After pulping, the pulp is discharged to a pulp chest 2. With a batch-type
pulping operation, the pulp chest serves as a storage facility for the
pulp, and the pulp is then discharged continuously from the pulp chest
through the remainder of the papermaking process. When a continuous
pulping process is employed, the pulp chest will serve to compensate for
variations in flow from the pulper 1 and provide a consistent flow
throughout the papermaking process.
The pulp is then pumped from the dump chest 2 to a pulp cleaner 3, which
acts to remove suspended solids and particulate material from the pulp.
The cleaning operation is, in itself, conventional and consists of a
series of sequential cleaning procedures. More particularly, large and
heavy debris, such as bark, stones, glass, metal pieces, and the like, are
initially removed from the pulp slurry, preferably by a liquid cyclone.
The pulp then undergoes a coarse screening operation to remove
larger-sized contaminants. Following the coarse screening, the pulp is
subjected to a finer secondary screening, in which the pulp is subjected
to a shearing action, and smaller sized solids or contaminants are
removed.
After the secondary screening, the pulp slurry then undergoes a forward
cleaning to remove small particles of sand, fiber bundles, ink, and the
like. At this stage the pulp generally has a consistency of about 1.5%
solids, and a liquid cyclone is employed for the forward cleaning, with
several stages being utilized. The solid particulate material removed from
the pulp during the forward cleaning is then transferred to a compactor
and the liquid resulting from the compaction process is discharged to a
clarifier 4.
Following the forward cleaning the pulp slurry is passed through a slotted
screen cleaner which acts to remove plastics, hot melt adhesives, and the
like from the slurry. The residue from this cleaning operation is
compacted and the liquid recovered from the compaction operation again is
transferred to the clarifier 4.
At this stage the pulp may still contains small quantities of wax, latex,
hot melt adhesives, and the like, and the pulp is then subjected to a
reverse cleaning operation in a liquid cyclone. In this operation, the
lighter weight materials, such as wax, latex, and the like, are removed
from the heavier pulp, and again the reverse cleaning can be carried out
in a plurality of stages. The lighter phase, which is removed during the
reverse cleaning, is also transferred to the clarifier 4.
The clarifier, in itself, is a conventional type and is preferably a
floatation clarifier. The upper floating portion of the material in the
clarifier is in the form of a sludge and has a solids content in the range
of about 1.5% to 2.5%. The sludge is conveyed to a solid waste dewatering
mechanism 5, while the liquid effluent from the clarifier 4 is transferred
to an evaporator 6.
In a typical papermaking process, steam from a steam generating plant is
supplied to the dryers in the dryer section of the papermaking machine.
The typical papermaking machine may contain approximately twenty to one
hundred dryers and the paper web supported by a dryer fabric is passed
over the dryers to dry the paper. It is customary to use a cascading steam
system in the dryer section of the papermaking machine, in which high
pressure steam at a pressure of about 160 psig, is supplied to a first
group of dryers and lower pressure steam discharged from the first group
of dryers is supplied to a second group, of dryers. This cascading system
is continued, so that low pressure steam at perhaps a pressure of 50 psig,
along with steam condensate, is discharged from a last group of dryers in
the section and either condensed and returned to the steam generating
plant, or discharged to the atmosphere.
In accordance with a feature of the invention, low pressure steam
discharged from the dryer section of the papermaking machine 9 is utilized
as the power source to evaporate the liquid effluent from the clarifier.
In this regard, the steam, at a pressure of perhaps 50 psig, and
condensate from the dryers is discharged to a condensate tank where the
steam is separated from the condensate. The condensate can then be
employed as a heat source to heat incoming process water to the
papermaking machine. The low pressure steam from the condensate tank, at
perhaps a pressure of 45 psig, is supplied to evaporator 6 to evaporate
the liquid effluent. The evaporation is preferably carried out by a
multiple effect evaporator, such as that sold by Goslin-Birmingham of
Birmingham, Ala. The low pressure steam from the dryer section is utilized
to heat the liquid effluent to produce water vapor and organic vapors, due
to the fact that the effluent may contain small amounts of volatile
organic constituents. The vapor resulting from the evaporation of the
liquid effluent is then condensed and transferred to a water storage tank
7. The condensed dryer steam from evaporator 6 can be combined with the
condensate from the vaporized liquid effluent in tank 7, or alternately,
as the condensate from the dryer steam is relatively pure, it can be
recycled to the steam generating plant, as shown in the flow diagram.
While a typical papermaking operation utilizes a cascading steam system in
the dryer section, a non-cascading system is preferred in the invention,
with high pressure steam being supplied to all of the dryers in the dryer
section. The reason this is preferred is that all dryers may be maintained
at maximum pressure, which maximizes the drying rate and therefore
minimizes the investment in drying equipment. A non-cascading system is
economically feasible with the process of the invention because the steam
being discharged from the dryer section is supplied to the evaporator 6
and used as a heat source for the evaporation of the liquid effluent from
the clarifier.
The evaporated residue of crystals, if an evaporation is employed, may have
a solids content of 40% to 60% by weight. It is possible, and preferable,
to not concentrate the residue to these high levels, since this may result
in evaporator fouling. It is only necessary to evaporate a portion of the
contaminated water in the processing loop, upstream or ahead of the pulp
washer, so as to supply the necessary quantity of condensate critical end
uses, such as pump seals, etc. The solids content in the water system
upstream of the pulp washing will rise to an equilibrium at which point
the removals with the sludge will be equal to the rate of addition with
the wastepaper.
In the dewatering unit 5, the solid waste is dewatered by mechanical
equipment, such as a compactor, screw press, or belt press and the small
amount of liquid removed from the solid waste can be returned to the
clarifier. The dewatered sludge having a consistency generally in the
range of about 35% to 50% by weight solids, can then either be landfilled
or incinerated. With proper design of the dewatering unit 5, the colloidal
material will be entrapped by the filter cake and the ionic solution in
the filter cake increases as the lower ionic solution is displaced. The
incineration of the dewatered residue can be accomplished either in a coal
or oil fired boiler. The dewatered residue is high in fuel value and also
contains sulfur capturing sodium hydroxide and calcium carbonate. Applying
the residue to coal as a dust control has a positive impact on sulfur
dioxide emissions from the furnace or boiler.
The dewatered residue can also be used for dust control of fly ash. Fly ash
from coal burning operations is normally acidic, since the coal will
contain high levels of sulfur. By utilizing the residue for dust control,
the pH can be maintained closer to neutral.
As shown in the flow diagram the pulp slurry after the cleaning operation
and having a solids content of about 0.4% to 2.0% by weight of solids, is
pumped to pulp washer 8, in which the dissolved solids contained in the
fiber supply, or generated through the pulping operation, are washed from
the pulp. When dealing with recycled pulp, the washing operation can also
remove dirt, fillers, or suspended solids, such as clay and calcium
carbonate. The pulp washing can be carried out by a method as disclosed in
U.S. Pat. No. 5,599,426, which is incorporated herein by reference. As
disclosed in that patent application, a very thin mat of pulp is supported
between a pair of porous endless belts and passed in a sinuous path over a
series of rolls. Wash water is impinged against opposite faces of the mat
and the tension in the belts as they pass over the rolls creates a
dewatering action, so that the pulp is alternately showered with water and
then dewatered by the belt tension, thereby removing fillers and dissolved
solids from the pulp mat.
With the use of pulp produced from recycled material which contains ink,
deinking and ink removal operations are required. The deinking can be
accomplished by pulping the recycled material with dispersant chemicals,
such as a surfactant, that act to dissociate the ink from the fibers and
disperse the ink particles in the aqueous pulp. During the pulping
operation, depending upon the nature of the ink, sodium silicate can also
be incorporated with the surfactant during the pulping. The sodium
silicate provides alkalinity and aids in releasing and dispersing the ink
particles. In addition, hydrogen peroxide and chelating agents can be
utilized during pulping, which aid in solubilizing certain oils in the
ink, act as a brightening agent, and permit highly alkaline operations
without color reversion. Through this treatment during pulping, the ink is
dissociated from the cellulosic fibers.
Following the washing operation, the recycled pulp can then be subjected to
an ink removal operation. Ink removal can be achieved either by froth
floatation or dilution washing, which can include a side hill screen
washer, a gravity decker, a vacuum filter, an inclined screw extractor, or
other types of washing. The publication "Beloit Corporation Deinking
Manual" 2nd Ed., 1979, describes conventional ink removal processes.
After deinking the recycled pulp may have a dingy color due to a small
portion of residual ink, and thus the pulp is normally subjected to
bleaching. In the bleaching operation, the pulp is thickened to a
consistency of about 15% solids, and bleached by adding a material, such
as hydrogen peroxide, ozone, or oxygen. The pulp is thickened in order to
reduce the amount of bleaching chemicals that are required.
The pulp then undergoes stock preparation, which normally consists of
refining and dilution to provide the pulp in the headbox of the
papermaking machine 9, with a consistency of about 0.2% to 1.2% by weight
solids.
In the papermaking process, the pulp as a thin web or layer is conveyed
through the forming, press and dryer sections of the papermaking machine 9
in a conventional manner to form the paper sheet.
In the papermaking operation, relatively pure water, having a minimum
mineral content is required for certain equipment, such as the showers,
and vacuum pump sealing water. In accordance with the invention, the
condensate from storage tank 7 is used in the papermaking operation, as
shown in the flow diagram. Depending on the water balance, make-up can
also be added to storage tank 7. The makeup water would normally be
municipal water, and if the makeup water has a high mineral content, the
makeup water can be added to the liquid effluent flowing between the
clarifier 4 and the evaporator 6, so that the makeup water is then
subjected to the evaporation process.
Water recovered from the papermaking operation, which can consist of water
recovered from the showers and pump seal water, as well as water extracted
from the wet paper web, is then recycled to the pulp washer 8 and used as
the wash water for washing the pulp. The water recovered from the pulp
washing operation, which consists of residual wash water, as well as water
recovered by virtue of the increase in consistency of the pulp as it
passes through the washing process, is then recycled to the pulper 1 for
use in producing the pulp slurry. In addition, a portion of the
recirculated water as shown in the flow diagram, can be recycled to the
pulp cleaner 2.
The invention maintains two distinct water systems. One water system is
maintained upstream or preceding the pulp washer 8 and includes the
pulping vessel 1, pulp cleaner 3, clarifier 4, and dewatering unit 5. This
water system contains a relatively high level of dissolved solids. The
second water system located downstream of the pulp washer 8 is low in
dissolved solids. The wash water supplied to the countercurrent pulp
washer 8 is excess papermaking water and the water recovered from the pulp
washing is utilized as make-up water in the pulping vessel.
In the pulp washer 8, the recycled pulp is subjected to counter-current
washing. Normally recycled pulp is not washed in a counter-current system.
However, the counter-current washing acts to remove the majority of the
dissolved solids in the pulp through use of a relatively small volume of
water, thus preventing the dissolved solids, as well as colloidal
material, from entering the paper machine water system, thus maintaining a
proper balance between the pulping water system and the paper machine
water system, so that there is no contaminated water discharge from the
overall system.
The process of the invention provides a papermaking operation which has
zero water discharge and the solids discharged from the process have a
consistency such that they can be either landfilled or incinerated.
The process also provides a higher fiber yield, and as there is no fiber
loss in a water discharge as in conventional processes. In many
conventional papermaking operations, a substantial portion of the small
fibers or fines are lost in the liquid discharge of the operation.
As a further advantage, low pressure steam discharged from the dryer
section of the papermaking machine is utilized as the heat source to
evaporate the liquid effluent from the clarifier, and the condensate from
the evaporation process serves as a heat source for the incoming water to
the papermaking machine. Thus, it is not necessary to directly heat the
incoming water as is required in a conventional papermaking operation.
The process of the invention also enables brackish water having a high
mineral content to be employed as make-up water by introducing the makeup
water ahead of the evaporator, so that the makeup water is also vaporized.
As the process has zero liquid discharge, the process utilizes
substantially smaller quantities of water than a conventional papermaking
operation, and this has distinct advantages in arid regions of the country
where water is at a premium.
FIG. 2 illustrates a modified form of the invention. As in the first
embodiment, a recycled cellulosic pulp slurry is initially produced in a
pulping vessel 10 using conventional techniques. As previously described,
the recycled cellulosic material can include used corrugated containers,
waste paper, white office waste, or the like, and normally contains debris
and solid contaminants. The recycled material also may include a
substantial quantity of water insoluble fillers, such as clay and calcium
carbonate, as well as ink. In addition, the recycled cellulose may also
contain residual water soluble materials emanating from the original
pulping or papermaking operation, such as sodium salts of lignin, alum,
starch and the like. In the pulping operation, pulping chemicals, such as
sodium hydroxide, are added to the pulp along with water and conventional
surfactants. Sodium silicate can also be used as a deinking aid and
hydrogen peroxide as a brightener. During the pulping, the pulp is
maintained at a temperature generally in the range of 120.degree. F. to
160.degree. F. and is subject to agitation or shearing to break down the
fiber bundles.
As described with respect to the first embodiment, the pulp slurry, having
a consistency generally in the range of 2% to 5% by weight of solids, is
discharged from the pulping vessel to a pulp chest, not shown, and is then
pumped from the pulp chest to a pulp cleaner 11. In the pulp cleaner, the
pulp slurry is subjected to a conventional cleaning procedure to remove
suspended solids and particulate material from the pulp. As described with
respect to the first embodiment, large and heavy debris is initially
removed from the pulp slurry by centrifugal cleaning as, for example, by
using a liquid cyclone. The pulp then undergoes a coarse screening to
remove larger sized contaminants, followed by a finer secondary screening
to remove smaller solids or contaminants.
As previously described, after the secondary screening, the pulp slurry
then undergoes several stages of forward cleaning forward followed by
several stages of reverse cleaning. Following the reverse cleaning, the
pulp slurry can be subjected to a conventional floatation clarification to
remove lighter weight contaminants.
Thus, the cleaning operation, after removal of the heavy debris, produces
two aqueous flow streams, the first flow stream consisting primarily of
suspended cellulosic fibers that are substantially free of solid
contaminants, along with water soluble dissolved solids. In practice, this
pulp flow stream which is pumped to the pulp washer 12 has a consistency
generally in the range of about 0.5% to 3.0% solids.
The second or reject flow stream being discharged from the cleaning
operation consists of rejects from the coarse and fine screening, rejects
from the forward and reverse cleaning operations, as well as rejects from
the floatation clarification. Thus, this reject stream consists primarily
of suspended solids contaminants, along with dissolved solids. In
practice, the reject stream may have a consistency in the range of 2.0% to
2.5% solids.
The cleaned pulp stream from the pulp cleaner 11 is pulped to the pulp
washer 12, wherein the pulp is washed to remove dissolved solids that
originated from the recycled fibrous supply, or generated through the
pulping operation. As described in the prior embodiment, the washing
operation can also remove residual dirt, fillers, or suspended solids,
such as clay and calcium carbonate, when dealing with recycled pulp. The
washing is preferably a countercurrent type and the washing can be carried
out by a method as disclosed in U.S. Pat. No. 5,599,426, which is
incorporated herein by reference. As previously described, the washed,
recycled pulp can then be subjected to an ink removal operation, and then
bleached, if desired, by a bleaching agent, such as hydrogen peroxide,
ozone, or oxygen. The pulp then undergoes stock preparation consisting of
refining and dilution to provide the pulp in the head box of the
papermaking machine 13 with a consistency of about 0.2% to 1.2% by weight
solids.
In the papermaking process, the pulp as a thin web or layer is conveyed
through the forming, press and dryer sections of the papermaking machine
in a conventional manner to form the paper sheet.
Makeup water, which normally would be municipal water, is supplied to the
papermaking machine 13, for use as showers, pump seal water, and the like.
Water recovered from the papermaking operation, which can consist of water
removed from the showers and pump seal water, as well as water extracted
from the wet paper web, is then recirculated to the pulp washer 12 and
used as the wash water for washing the pulp. As shown in FIG. 2, the water
recovered from the pulp washing operation, as well as water recovered by
virtue of the increase in consistency of the pulp as it passes through the
washing process, is then recycled to the pulping vessel 10 for use in
producing the pulp slurry. A portion of the recirculated water, as shown
in FIG. 2, can also be recycled to the pulp cleaner 11.
The reject flow stream being discharged from the pulp cleaner 11 normally
has a solids content of about 2.0 to 2.5% by weight solids, and is
delivered to a dewatering unit 14 where the material is dewatered,
preferably by mechanical equipment, such as a compactor, screw press, or
belt press. The resulting dewatered sludge, which includes the solid
contaminants as well as a portion of the dissolved solids. has a
consistency generally of about 35% to 50% by weight solids and can either
be land filled or incinerated. The water extracted during the dewatering
operation, which also contains dissolved solids, can be recycled to the
pulp cleaner 11, as shown in FIG. 2.
One of the primary concerns in a zero liquid discharge papermaking
operation is the gradual buildup of dissolved solids in the water phase. A
buildup of dissolved solids can adversely affect the physical properties
of the paper being produced. With the system of the invention, a
substantial portion of the dissolved solids are removed in the cake or
sludge produced by the dewatering unit, so that a control is provided for
the buildup of dissolved solids in the water phase.
With the method of the invention, as shown in FIG. 2, additional treating
operations, such as evaporation, reverse osmosis or ultrafiltration to
remove dissolved solids from the water phase are not required. However,
the overall papermaking operation can be enhanced, if desired, by
subjecting a portion of the water effluent from the dewatering unit 14 to
a purification unit 15, as shown by the dashed lines in FIG. 2. The
purification unit 15 serves to remove dissolved solids from the effluent
and can constitute an evaporation, reverse osmosis or ultrafiltration
system. The purified water from unit 15, which in the case of an
evaporation system would be condensate, is delivered to a water storage
unit 16 where it is combined with make-up water and fed as needed to the
papermaking machine. The solid residue from unit 15 can be landfilled or
incinerated, as shown in FIG. 2. By utilizing the water purification unit
15, additional purified water is made available for the papermaking
operation and this, in turn, increases the available water for washing
If the make-up water is of low quality, i.e. has a high mineral content,
the low quality water can be supplied to the purification unit 15, along
with the liquid effluent from the dewatering unit 14, rather than being
supplied to the water storage unit 16, as shown in FIG. 2. The purified
water from purification unit 15 is then delivered through water storage
unit 16 to papermaking machine 13.
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